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Cisco has issued an alert warning that a vulnerability in a commonly used routing protocol can be used to bring down service provider's core Internet infrastructure.

The vulnerability concerns BGP (Border Gateway Protocol), which is used for exchanging routing information between gateway host on the Internet, and can be exploited to create network outages.

Cisco said the issue came to light because of a malfunction in the BGP implementation of another (unnamed) vendor, which caused a series of crashes but it admits the problem might also affect its own kit.

The issue concerns as aspect of BGP that refers to attributes that describe a path from a source to a destination. It's been discovered that an unrecognised transitive attribute can result in memory corruption resulting in the immediate crash or subsequent failure of Cisco routers. The vulnerability is particular to certain, but common, configurations.

In a strongly worded notice, Cisco warned: "The vulnerability can be exercised repeatedly, affecting core routers, creating widespread network outages. This vulnerability can only be exercised in configurations that include both BGP and inbound route filtering on affected software."

Cisco is not aware of any malicious exploitation of the vulnerability and indeed it seems doubtful whether s'kiddies have enough knowledge of esoteric routing protocols to do much immediate harm.

Still there is always the possibility that an exploit package might be written and given the potential impact of the flaw - which could bring an entire network to its knees - service providers should address the issue sooner rather than later.

The vulnerability affects a wide variety of Cisco routers running BGP but not the networking giant's Lan switching kit. Cisco has advised customers of affected kit to update their software and further information is available here. ®